Since the advent of the automatic weapon, gunsmiths have sought a safe, reliable, and efficient mechanism for readying the action of a firearm quickly after a discharge. In the 1930s, the M1 Garand semi-automatic rifle was introduced. It was a gas-operated, self-loading weapon that proved to be very successful on the World War II battlefield. The Garand operated by taking a sample of expanding, hot, high-pressure gas from a fired cartridge and using that gas to push a small piston in a cylinder suspended under the barrel. This piston, in turn, pushed an operating rod to the rear, thereby rotating and unlocking the bolt. Residual energy in the rifle then pushed the operating rod and bolt fully to the rear of the stroke length, which allowed the expended brass casing to be ejected and, on the return to battery, chamber a fresh cartridge and lock the bolt, thus readying the weapon for the next shot. If the Garand had a major flaw, it was the unwanted muzzle jump produced by the weight of the entire firearm plus the large reciprocating weight. Muzzle jump produced in this way was especially problematic for weapons that are select fire, i.e., fully automatic.
In the 1950s, a new operating system was developed that used the energy from the expanding gas to rotate and open the bolt directly, without the reciprocating weight of a heavy operating rod. This, along with the utilization of lighter components made from aluminum and plastic, produced a weapon of lighter weight and better accuracy. These improvements came with a concurrent flaw, however, as along with the pulse of high-pressure gas to the bolt and bolt carrier came heat and carbon. In the early Vietnam War, this this resulted in M16 rifles jamming in combat conditions. At that time, different propellants were introduced, along with more frequent maintenance routines, to prevent this problem. In relatively recent years, there have been a number of patents granted for devices that convert M4-family direct gas rifles to a forward position and operating rod style operating system. Most recently, with respect to semi-automatic weapons, and firearms in the M4 family in particular, two camps of development emerged to address this issue: a first camp extolling direct impingement operating systems, and a second championing gas-piston operating systems. Both systems have unique advantages. However, the existence of drawbacks for each system still makes neither solution a perfect one.
Very basically, direct impingement operating systems route a portion of the combustion gas into the bolt carrier to cycle the action. When an operator fires the weapon, the trigger is depressed, causing the firing pin to impact the primer of a cartridge. The primer ignites, causing the ignition or propellant within the cartridge to explode. Combustion gas is thereby produced, forcing the bullet out of the cartridge casing and down the barrel. Combustion continues as the bullet travels down the barrel, producing further combustion gas. The bullet passes a gas port formed in the barrel, and a metered amount of the combustion gas passes into the gas port and back through a gas tube toward the rear of the rifle. The gas tube terminates above the bolt carrier at the gas key, thereby allowing the combustion gas to enter and directly impinge the bolt carrier, causing the bolt carrier to slam rearward, unlocking the bolt with a caroming action. The bolt carrier group then continues fully to the rear of the receiver, ejecting the spent cartridge and compressing the recoil into a spring. The spring urges the bolt carrier forward again, stripping a fresh cartridge from the magazine, and the action is cycled.
Direct impingement systems are efficient in terms of weight. They require no additional moving parts and essentially only an additional thin metal tube fixed along the barrel. However, these systems have at least one serious problem: they rely on the application of hot and dirty combustion gases directly back into the action of the firearm, thereby fouling the action. Carbon combustion particles, contaminants, moisture, and lubricants combine to leave deposits in the bolt carrier, preventing the bolt carrier from moving efficiently, effectively, and reliably with each shot. Direct impingement systems also suffer from the delays of a forced cool down time: the application of hot gas into the bolt carrier means that sometimes the firearm will overheat if not cooled. Both of these issues are irksome at the firing range, but are life threatening on the battlefield.
As a result of this recognized fouling problem, the gas-piston operating system has been revisited in recent years. Gas-pistons eliminate the additional application of combustion gas into the action, instead relying on a reciprocating rod that acts on the bolt carrier. A gas-piston-operated M16, AR15, or M4-pattern firearm, for example, includes a drive rod mounted for reciprocation above the barrel. The gas port forces combustion gas into a cylinder in the gas block containing a piston, which in turn is mechanically connected to a drive rod. The rear end of the drive rod is operatively coupled and positioned to push against a raised abutment on the bolt carrier to slam into it and push it rearward. This extracts the spent cartridge, and a spring urges the bolt carrier forward as with the direct impingement system.
While the gas-piston operating system solves the problem of fouling the bolt carrier, it presents the problem of a larger reciprocating mass on a precision weapon. The weight of the drive rod, and the speed with which it moves, affects the accuracy of the firearm. Earlier solutions pursued lighter constructions, but were more prone to failure. In addition, offset mechanical forces operating the bolt carrier present additional wear and jamming problems for the bolt carrier; some manufacturers have added roller systems to the bolt carrier to prevent scoring on upper receiver. Still further, the recoil caused by the reciprocation of the drive rod can wear on the operator over the long term, and in the short term, makes maintaining accuracy from shot to shot challenging: each time the weapon is fired, aim is slightly lost. Again, this can be cumbersome at the firing range, deadly on the battlefield.
Therefore, both direct impingement operating systems and gas-piston operating systems have their flaws. While each solves a problem, each presents one as well. An improved operating system which avoids all of these issues, and creates no new ones, is needed.